The production of fibrous, webbed products whereby fibers are laid down on a moving foraminous belt from a mixture of air is well known. Apparatus for the production of such webbed products generally comprises a fiber distributor having an inlet means therein for introducing the air-fiber stream, a permeable bottom wall and stirring means which move parallel to the bottom wall. A foraminous forming wire is positioned immediately below the fiber distributor and a suction box is situated below the wire in order to draw down the fibers onto the wire and hold them in place. The wire normally is supported along its length by rollers which are positioned below it and in the suction box, especially when a plurality of distributors are employed and each distributor has a plurality of stirring means associated therewith. Such an area of the apparatus is usually called the forming head. In order to insure the complete association of the fibers which are laid-down on the wire, it is oftimes desirable to introduce a binder with the air and fibers admitted into the distributors. These binders are normally added in the form of particles although they may also be used as liquids which are either sprayed onto the individual fibers or the web per se after it is formed, see U.S. Pat. No. 4,014,635.
In apparatus of the type described above, it is necessary to control a great variety of different variables in order to produce webs of optimum physical characteristics across their entire cross-section, such as basis weight, etc. One problem which has been continually faced in producing webs of this type is that the forming rate varies in the machine direction under the forming head in relation to the suction box during the continuous operation of the apparatus to the extent that the density, basis weight, surface properties, etc. of the formed web are not uniform. The forming rate is a direct function of the air velocity through the foraminous wire which, in part, is a function of the suction box. As the web is formed, the thicker it becomes, and the less air passes through it. Attempts to control the air rate through the web have included modifying the suction box so that more draw is created at the end thereof over which the thicker section of the web is passing. When a plurality of distributors are employed, such modifications of the suction box do not provide the control of air flow necessary to create a web with a substantially ripple-free surface, constant basis weight, cross-section density, etc.
Furthermore, when a solid binder is used, the draw of the suction box tends to pull particles thereof through the web and into the box where the rollers supporting the foraminous wire become contaminated. This contamination affects the efficiency of the rollers and also disturbs the path of the foraminous wire passing thereover.